We are investigating the formation and stability of hybrid double alpha-helices; i.e. double alpha helices in which the two associated chains are different. In particular, the equilibrium concentration of hybrids composed of one alpha and one beta chain of rabbit tropomyosin and of hybrids of rabbit and earthworm tropomyosins are being measured by a method developed previously. Thermal stabilities of double helical molecules in sodium dodecyl sulfate solutions are also being measured.